1. Field of the Invention
This invention relates to a fuel pump, and more particularly to a fuel pump for delivering high pressure fuel to a fuel injection system of an internal combustion engine. The preferred embodiment of the present invention is particularly suitable for supplying high pressure fuel to an accumulator or directly to the common rail of a common rail fuel injection system, but the invention is not limited to this application.
2. State of the Art
There are various characteristics which are recognised by those skilled in the art as being desirable in fuel pumps for fuel injection systems, particularly high pressure fuel pumps for use in common rail fuel injection systems. One such characteristic is the presence of a low dead (unswept) volume associated with the or each pumping plunger.
In order to minimise the dead volume associated with a pumping plunger it is known to obviate the need for an inlet valve at the discharge end of the plunger cylinder by providing a fill passage which opens into the cylinder immediately above the position of the end of the pump plunger when the pump plunger is at its bottom dead centre (bdc) position. With such an arrangement, as the pump plunger moves away from its bdc position it covers the fill passage and permits pressurization of the fuel trapped between the pump plunger and the discharge end of its associated cylinder. During the return stroke of the pump plunger a discharge valve prevents reverse flow of the fuel and a vacuum is created within the pumping cylinder until the fill passage is again uncovered by the pump plunger whereupon fuel may flow into the cylinder under the combined influences of the feed pressure in the fill passage and the vacuum created by the previous plunger movement. Whilst this design offers the advantage of a very small dead volume, it does suffer from the disadvantage of requiring a substantial force to move the plunger from its top dead centre (tdc) position to its bdc position because of plunger inertia and the vacuum which is created within the pumping cylinder during such movement. In known designs, a spring is used to move the plunger from tdc to bdc. The provision of this spring limits the design of the pump, and in particular, limits the ability of the designer to increase the stroke of the pump plunger to increase the output of the pump. This is because it is not possible to engineer a spring which can provide sufficient force, stroke and velocity characteristics at high engine speeds if the stroke of the pump plunger is too long.